The present invention relates to the construction of road and motorway infrastructures, such as base courses and foundation courses of roads, or of civil engineering infrastructures, such as rafts for building foundations, for airport parking or storage areas, for railway platforms or for port quays or wharfs. Generically, this collection of uses will be referred to hereafter by the term xe2x80x9cinfrastructuresxe2x80x9d.
It is well known in the manufacture of these infrastructures to use mixes of aggregates, of a hydraulic binder, comprising a slag and an activator, and of water (the gravel-slag or sand-slag technique). Next, the mix is spread out over the surface to be treated and is compacted according to particular specifications.
The aggregates consist, in a known manner, of grit or sand, for example, coming from quarries.
The slag used is a by-product of the manufacture of pig iron in blast furnaces and is consequently inexpensive. It is essentially composed of three oxides, namely CaO, SiO2 and Al2O3, which may be in the form of oxides, silicates, aluminates or aluminosilicates. On leaving the blast furnace, the slag is vitrified and made into particulate form, essentially using two methods. According to the first method, the slag leaving the blast furnace at 1500xc2x0 C. is sent, in the form of a fluid stream, into a tank whose walls include water jets; the slag forms particles having surface asperities: it is called xe2x80x9cgranulatedxe2x80x9d slag. The particles of granulated slag have dimensions of less than 5 mm, their average size generally being about 0.5 mm; the granulated slag contains approximately 1% of fines having a particle size of less than 80 micrometers. According to a second method, the slag is cooled by mechanically and hydraulically spraying it at high speed into the air; the particles are in the form of nodules having a spheroidal shape and a foam structure; it is called xe2x80x9cpelletizedxe2x80x9d slag. The particles of pelletized slag have a particle size of less than 80 mm, 95% of particles generally having average sizes of less than 20 mm; the pelletized slag contains virtually no fines.
The granulated or pelletized slag is generally used in preground form. By pregrinding, the particle size of the entire product is reduced and the proportion of fines of less than 80 xcexcm is increased.
It is also known to add an activator in pulverulent form to the slag. The activation of the vitrified blast-furnace slag consists, firstly, in the presence of water, in attacking the slag with a basic agent, the purpose of which is to dissolve a small amount of slag. Thus, by successive crystallization from the aqueous phase, solid hydrated compounds are formed, which results in the mix setting and hardening. The activator is defined in French Standard NF P 98-107; it may consist of lime, calcium sulphate or both these, it may also contain soda or potash, silica and also a product having, in addition, hydraulic binder properties.
The mix of aggregates, slag, activator and water must principally have the following properties.
Firstly, the mix must not set immediately, so as to be able to be spread out over the ground. It is desirable to have a workability time at least greater than 12 hours, generally between 15 and 30 hours. When the mix is spread out over the ground, it is compacted using a specified process and to a specified value. Setting then starts and may last 4 to years, the mix after compacting nevertheless being already sufficiently load-bearing in order for it to be possible, in particular, to drive over it immediately.
Secondly, the compacted mix must comply with mechanical performance standards. The mechanical performance characteristics taken into account are the 360-day tensile strength (Rt) and the 360-day elastic modulus (E), the values of which are estimated from test pieces manufactured in the laboratory. It has been found that the 360-day mechanical performance characteristics depended on the particle size of the granulated or pelletized slag and that it was favourable to use a preground slag, the particles of which are smaller than 2 mm and contain approximately 10% of fines.
However, the storage properties of the slag depend on the composition of the latter. This is because, when an iron ore coming from mines in Lorraine (minette ore) is used in blast furnaces and worked according to the Thomas process, a stable and storable preground slag is obtained and it is possible to achieve 360-day mechanical performance characteristics according to the standards, even after the preground slag has been stored. However, when the slag comes from the treatment of another ore, for example, haematite, the preground slag very often does not comply with the mechanical performance standards; this is because it has been found that, during storage, the proportion of fines decreases in a random fashion during storage; it may thus go from 10 to 4% in a few hours.